First of all, there needs to be a need for a new word, a semantic space in the language that needs to be filled. Lets use some of Miltons words as examples. His day, like ours, had a lot of worship wars in the Church of England. The word liturgy existed. But, earlier, that was pretty much the only kind of worship there was. There was a need for an adjectival form of that word to distinguish that type of worship from the alternatives. So Milton turned the existing noun into an adjective by adding a Latin adjectival ending. Hence a new word that we use today in our own worship wars: liturgical.

An even better, because more poetic, example: The new Copernican cosmology meant that the earth and the planets spin around in a vast void. In Paradise Lost, Milton needed to write about Satan flying to earth. Dante in the Middle Ages had imagined Hell as existing in the center of the earth. Milton imagines it more like another planet. The word space existed to refer to expanse, area, extent. Milton took that word and made it refer to the realm beyond earths atmosphere. Satan flew through space. What great poetry! Imagine hearing that poetic image for the first time. But now we have a new word, one that names something that was nameless before.